Peter, please do not take your son and his friend diving if they have only just qualified. The Solent / Isle of Wight is not the place to learn by making mistakes, strong tides and poor vis will only scare them off an excellent past time and missing divers under water is a supervisors worst nightmare.
Can I suggest instead you find your local British Sub Aqua club and take them along. Let the club organise the diving safely and show them the local waters. by all means offer to help the club by using your rib which is bound to be appreciated, but safe diving requires experienced dive leaders in the water and an experienced diver acting in the supervisor role on the surface to organise a safe dive, which will minimise risks.
Have a good look at this:
Or alternatively have a chat with Andark who organise safe open water dives for small groups on a regular basis, they are a PADI school.